Fuel cells have been gaining much attention in recent years as a power source with good operation efficiency and environmental characteristics. A fuel cell is a device that generates electricity through an electrochemical reaction of hydrogen and oxygen. A fuel cell outputs electric power as required by controlling the amount of fuel gas supplied. Sometimes, response of output electric power becomes poor due to a response delay of the gas supply amount. As one means for avoiding this problem, there is proposed a technique in which a fuel cell and a battery are connected in parallel to constitute the power source. For example, Japanese Patent Application Laid-Open No. 2000-12059 discloses a technique in which the output voltage of the fuel cell is converted by a DC-DC converter to allow usage of a battery and a fuel cell in combination. However, in the above-mentioned structure, further improvement of electric power output efficiency was desired.